The first trimester lasts 13 weeks and 6 days, starting from the first day of your last menstrual period. That means it covers weeks 1 through 13 of pregnancy, even though conception itself doesn’t happen until around week 2 or 3. This counting method can feel confusing, but it’s the standard used by doctors because most people can pinpoint their last period more reliably than the exact day of conception.
How Pregnancy Weeks Are Counted
Pregnancy dating begins on the first day of your last menstrual period (LMP), not the day you actually conceived. Since ovulation and fertilization typically happen about two weeks into your cycle, you’re already considered “2 weeks pregnant” at the time of conception. By the time you miss a period and get a positive test, you’re usually around 4 to 5 weeks along.
This is why the first trimester spans roughly 14 calendar weeks on paper but only includes about 12 weeks of actual embryonic and fetal development. The American College of Obstetricians and Gynecologists defines the first trimester as the period from the first day of LMP through 13 weeks and 6 days. After that, you’re in the second trimester.
What Happens During Each Phase
Weeks 1 Through 4
Weeks 1 and 2 are technically pre-conception. Fertilization takes place in the fallopian tube, and over the next few days, the single cell divides into a cluster of cells that travels to the uterus and implants in the lining. By the end of week 4, implantation is complete and the body begins producing the pregnancy hormone hCG, which is what home pregnancy tests detect.
Weeks 5 Through 8
This stretch is when the most dramatic development happens. The heart, brain, spinal cord, and lungs begin forming. The lungs start building the tubes that will eventually carry air in and out after birth, and the inner ear begins to develop. By around week 6, a heartbeat can often be detected on ultrasound. hCG levels rise rapidly during this window, jumping from a range of 200 to 7,000 at week 5 up to 3,000 to 160,000 by week 7.
That hormone surge is also what drives many early pregnancy symptoms. Nausea typically starts around week 6, with most people noticing it before week 9. It tends to peak between weeks 8 and 10.
Weeks 9 Through 13
By week 9, the embryo is reclassified as a fetus. Cartilage for the limbs, hands, and feet begins forming, though it won’t harden into bone for several more weeks. Eyelids develop but stay closed. The genitals start to take shape, and the liver begins its own development. hCG levels hit their highest point during this stretch, peaking somewhere in the range of 32,000 to 210,000 before gradually declining through the rest of pregnancy.
For many people, nausea begins to ease around week 13, right as the first trimester wraps up. Fatigue often lifts around the same time, which is why the second trimester has a reputation as the more comfortable stretch of pregnancy.
First Trimester Screening and Testing
Most prenatal screening happens in a narrow window toward the end of the first trimester. Between weeks 11 and 13, you’ll typically be offered a combination of a blood test and an ultrasound called a nuchal translucency scan. The ultrasound measures a small pocket of fluid at the back of the baby’s neck, which can signal higher chances of certain genetic conditions. A non-invasive prenatal test (NIPT), which analyzes fragments of fetal DNA circulating in your blood, is also available during this same window.
Your first prenatal appointment usually happens around weeks 8 to 10, depending on your provider’s scheduling. That visit typically includes confirmation of the pregnancy, an estimated due date, bloodwork, and a medical history review. If you’re trying to time things, it helps to know these appointments cluster between weeks 8 and 13, so the last month of the first trimester tends to be the busiest for medical visits.
Why the First Trimester Feels So Long
Thirteen weeks is only about three months, but it often feels like the longest stretch of pregnancy. Part of that is physical: nausea, exhaustion, breast tenderness, and frequent urination are at their most intense during these weeks. Part of it is emotional. Many people wait until after the first trimester to share the news, which means navigating symptoms in relative secrecy while the most critical development is happening behind the scenes.
The good news is that by the time you cross into week 14, most of the foundational organ development is complete, hormone levels begin to stabilize, and the symptoms that defined your first 13 weeks start to fade.